1. Field of the Invention
The present invention relates to a method and apparatus for melt-spinning thermoplastic polymer fibers having high molecular orientation by extruding a fiber-forming polymer into a pressurized atmosphere and then taking it up under normal atmospheric conditions.
2. Description of Related Art
For obtaining a well-oriented fiber by the ordinary melt-spinning system, in which a thermoplastic polymer is melted, spun through a spinneret, and taken up at a constant speed after being quenched and oiled, the melt-spinning operation is preferably carried out under a high take-up speed. The increase of the take-up speed is the most effective for this purpose and the control thereof is easier than with other factors influencing the molecular orientation of the resultant fiber, such as the polymerization degree of the polymer, melting temperature, draft ratio, extrusion rate of the polymer per nozzle hole, or quenching conditions.
Of course, by properly determining, the melting temperature, a draft ratio, quenching conditions, and other spinning conditions, the degree of molecular orientation in the resultant fiber can be further improved. For example, a higher melting temperature, larger draft ratio, and rapid quenching speed may result in highly oriented fibers. In such spinning, however, the conditions are critical if one wishes to have a useful yarn having high uniformity and properties necessary for practical use. Further, control is very difficult.
There have been proposed, in Japanese Examined Patent Publication (Kokoku) Nos. 47-32130 and 47-33736, a method for spinning a molten polymer at a high spinning speed from a spinneret into a pressurized chamber disposed directly beneath the spinneret. In the chamber, the polymer is quenched to form fibers. Thereafter, it is ejected as a fully drawn fiber out from the chamber through a nozzle provided on the bottom of the chamber together with a flow of high pressure gas. This method aims to obtain a fully drawn fiber utilized as a material for making a non-woven fabric or web. The method, however, has a drawback that the control of yarn processing factors, such as yarn take-up speed, drawing force, or draw ratio is very difficult because the yarn is propelled mainly by a dragging force of the jet air.
Another method for spinning a polymer under a highly pressurized atmosphere has been proposed in Japanese Unexamined Patent Publication (Kokai) No. 50-71922, in which a yarn extruded from a spinneret is quenched under normal atmospheric pressure until just before it has reached a fully solidified state and then is taken up through a chamber pressurized above 0.1 kg/cm.sup.2 G. According to this method, however, sealing means for the yarn inlet and exit of the chamber are necessary. Particularly, in the area of the inlet, non-touch sealing is required because the yarn passing thereby is not yet fully solidified. Thus, the cross-section of the inlet must be large, whereby the sealing effect tends to be lowered. As a result, as described in the above publication, the interior pressure of the chamber can be elevated only to 0.7 kg/cm.sup.2 G. Under such a low pressure, high molecular orientation of the resultant yarn cannot be expected.